The invention concerns a method for transportation and storage of gas and oil, or gas and condensate.
In cases where pipelines for transportation of gas are absent, it is difficult to perform efficient exploitation of the gas. The gas can not be combusted continuously, it can not be used on the spot, for example on a platform offshore, and can not be carried to the consumer through a pipeline.
One possibility in such cases is re-injection of valuable gas back into the reservoir. Exploitation of single isolated gas fields, e.g., offshore, is economically without using the gas on the spot or transporting it through a pipeline. It has also been suggested to treat the gas on the spot in the production of liquefied natural gas such as, methanol and ammonia. The three latter alternatives, however, require comprehensive treatment of the gas and/or another gas product, and require complex equipment, in a scale which is unsuited for working offshore.
In an effort to solve problems of this type it has been suggested to produce gas hydrates. Gas hydrates form extremely structured systems, in which all water is bonded in an open crystal structure ("empty hydrate") through hydrogen bonds, wherein the hydrate has a density less than ice. The open nature of the crystal structure means, gas molecules can become immobilized and captured in the lattice cavities without breaking the hydrogen bonds. All hydrogen atoms (except for a few surface atoms) are involved in hydrogen bonds, and the gas molecules cooperate with the water molecules through van der Waals forces (non-polar). In this way the hydrate is a phase enabling polar bonding between water molecules and non-polar interaction between foreign molecules. For example, hydrates formed from on cubic meter of water may bond about 150-170 m.sup.3 methane. This fact has opened new possibilities for transportation and storage of hydrate forming gases.
For general examples of prior art from the patent literature within this technical field, we refer to U.S. Pat. No. 3,888,434 and NO Patent 149976. However, both concepts require use of high pressure to realize transportation over longer distances in order to avoid dissociation of gas and water from the hydrates.
NO Patent 172808 describes a method for production of gas hydrates in a gas phase reactor to facilitate transportation and storage of hydrate forming gases. NO Patent Application 944974 (published after the filing date of the present application), describes an alternative method for transportation of gas hydrate. Hydrate forming gases and water are supplied to a pressurized liquid phase reactor to form gas hydrate. In order to absorb the heat evolved from the exothermal hydrate-forming reaction, the reactor is supplied with a combined cooling and reactant medium comprising a slurry of ice crystals in a liquid carrier such as water or condensate of light hydrocarbon fractions. In the reactor, the ice crystals melt at least partially and absorb reaction heat from the exothermal hydrate forming reaction between gas and water, whereupon water and the carrier liquid are separated from the hydrate in a subsequent separation step. Gas hydrates produced in accordance with the two latter patent publications enable separate transportation of gas at or close to atmospheric pressure and at substantially adiabatic conditions at temperatures below 0.degree. C. A transportation of this type has evident advantages compared to transportation of LNG, for example, in longer distances and with hydrate processes found in the prior art.
For small oil fields, such as in the North Sea, with a low production rate and short lifetime, it is economically impossible to perform any exploitation since any oil exploitation also requires handling of the natural gas. With the prior art technology the transportation of oil and gas must occur separately, where the gas is treated and transported as LNG, as liquid products such as methanol or other chemical products, or as gas hydrates. This transportation requires separate storage and transportation units, and will, as mentioned above, render exploitation economically unattractive. On the other hand, the gas may be burned, or re-injected back to the reservoir, but then the economic potential of the natural gas will be lost.
Moreover, there are fields located at long distances from the terminal that require long distance transportation by tanker. In some cases one must resort to long distance pipeline transportation of the oil, either alone or in combination with gas in a two-phase flow, and in many cases the gas will be re-injected back to the reservoir. In such cases it is usually economically unattractive to perform any gas treatment to produce gas hydrate since there is no real transportation possibility for the gas hydrate. On the other hand, two-phase transportation of gas and oil requires high pressures and accordingly increased costs, whereas re-injection of gas back to the reservoir will forfeit a valuable resource.